A known way of constructing a steam turbine diaphragm is to mount an annulus of static guide blades between an inner ring and an outer ring. Each such blade comprises a blade unit in which an aerofoil portion extends between an inner platform and an outer platform, the blade unit being machined as a single component. This is known as the “platform” type of construction. Each platform is in the form of a segment of a cylinder so that when the annulus of blade units is assembled the inner platforms combine to create an inner port wall and the outer platforms combine to create an outer port wall. The inner platforms are welded to an inner ring that retains the turbine blades and provides a mount for a sealing arrangement, such as a labyrinth seal, that acts between the inner ring and a rotor shaft of the turbine. The outer platforms are welded to an outer ring that provides support and rigidity to the diaphragm. Each of the inner and outer rings usually comprises two semi-circular halves which are joined along a plane that contains the major axis of the diaphragm and passes between blade units so that the entire diaphragm can be separated into two parts for assembly around the rotor of the turbo-machine.
Existing platform constructions for HP or IP steam turbine diaphragms generally comprise solid inner and outer rings cut from thick metal plate, or forged, or formed from bar stock. Since such rings in large turbines have substantial dimensions in the axial and radial directions of the turbine, e.g., 100 mm to 200 mm, the cost of welding together the components of the diaphragm is a significant factor in the price of a large steam turbine, not least because the necessary deep penetration welds require advanced specialist welding equipment for their production. Furthermore, welds are a possible source of metallurgical defects in the diaphragm and it is also necessary to heat treat the diaphragm in order to relieve stresses caused by the welding processes.